Reference for Eruvin 167:11

אלא אמר אביי הכא בבור מליאה פירות עסקינן והא חסרי

- Since the use of the cistern is permitted<span class="x" onmousemove="('comment',' To the tenants of the gallery.');">²³</span> when full it is also permitted when some of the water is wanting. On the contrary! Since Its use would be forbidden when it is not full should it not also be forbidden when full?

Jastrow